What is DEBUG pin in debug UART connector?

[anverx]

Hi
I'm connecting mod-gps to debug port of oLinuxino Maxi.  UEXT UART is taken by something else. 

When i draw power from UEXT and connect TX/RX to DEBUG TX/RX, i receive the NMEA data and gpsd runs just fine. 

What i would like to do is to have a single 4 pin connector for Debug UART.  Debug UART port consist of 4 pins, TX/RX/Ground/Debug.  I'd like to draw 3v from debug pin to power the GPS. If it's a pin coming directly from the processor it should supply the needed 40mA.  But how would i set the Debug pin to a high state?

[Kean]

Page 32 of the user manual[1] shows it comes from processor pin 84 (a typo, it is actually 89)

Page 36-5 (1407) of the imx233 reference manual list this as the serial JTAG debug port, which is described in chapter 34, more particularly section 34.2.7.  From this you'll see it is not a user programmable output.

Also, note that even on normal I/O pins the maximum drive strength is 12mA, or 16mA/24mA on certain special pins (see 37.2.2.1).

So basically, you can't power the GPS from this single connector - you'll need to make up a Y cable of some sort.

Hope this helps.

[1] https://www.olimex.com/Products/OLinuXino/iMX233/iMX233-OLinuXino-MAXI/resources/iMX233-OLINUXINO-MAXI.pdf
[2] http://cache.nxp.com/files/dsp/doc/ref_manual/IMX23RM.pdf?fpsp=1&WT_TYPE=Reference%20Manuals&WT_VENDOR=FREESCALE&WT_FILE_FORMAT=pdf&WT_ASSET=Documentation&fileExt=.pdf

[anverx]

Thanks for clearing that out Kean.
Guess i'll have to stick to 2 and 2 pin connectors. 
Didn't realize there was so little current in processor pins, i somehow assumed it was around 50mA.
I actually tried flicking GPIO 84 on and off, but i wasn't sure if processor pin 84 is the same thing as GPIO 84, and, as you say 84 is actually a different pin entirely.

[anverx]

It looks like gps data in debug port is confusing the bootloader and device doesn't boot. Dammit.

[Kean]

Yes, I think certain keystrokes at boot could cause problems.
Could you use a spare GPIO to drive a transistor (MOSFET) that turns the GPS power on after boot?
Some GPS modules actually have a separate enable/sleep input that can be driven directly by a GPIO.

[anverx]

I'm using a custom expansion board, which luckily has a spare relay that i can use.  Is there any practical difference which line to run through the relay, GPS power or GPS TX ? 
Cutting the TX makes more sense to me because the GPS can then start together with the board.  GPS cold start takes a long time.  In my application the coordinates are only read once a few minutes, so it isn't very critical either way.

MOD GPS (as sold by Olimex) has only 4 wires in the cable: Power/GND/TX/RX, which makes sense since it has the UEXT connector.

[Kean]

Yeah, if you're not worried about power consumption (and don't otherwise need to restart the GPS), then switching the GPS Tx line seems sensible.